Although certain enzymatic markers have been associated with lymphoid cells, only a few seem to have relative specificity for thymus dependent (T) or thymus independent (B) cells. Based on the finding of high thymidine phosphorylase activity (300-600 units/mg protein) in various human B-cell lines as compared to T-cell lines (20-30 units/mg protein) we propose to evaluate the enzyme on clinical samples (separated and immunologically characterized cells from normal and various leukemic subjects) to determine its value as a marker for the characterization of B- cells from T-cells or for defining the stage of differentiation of hematopoietic cells. The results of this study are envisioned to be of value for the more accurate diagnosis of B- and T-cell neoplasms which differ in prognosis and in response to therapy.